GB1575397A - Radio lighthouse signal format - Google Patents

Description

(54) RADIO LIGHTHOUSE SIGNAL FORMAT (71) We, STANDARD TELE PHONES AND CABLES LIMITED, a British Company, of 190 Strand, London WC2R 1DU England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to apparatus for transmitting bearing information, and is particularly applicable for marine navigation in coastal waters.

There are many small fishing and other harbours where small boats have to make an approach or departure along well but narrowly defined paths to avoid dangerous obstructions. Very often the safe channel can be determined by reference to no more than a bearing upon some fixed point on the shore. However, there are a large number of small boats which use such harbours which are not equipped with specialised navigational aids such as radar, or even direction finding radio. Nevertheless most boats carry, or can be equipped with, low cost communications receivers, for example even a portable domestic transistor radio, capable of receiving VHF transmissions.

Such boats, lacking specialised navigation equipment, are unable to navigate in the approaches to many small harbours after dark, unless there is a lighthouse, especially in hostile weather conditions, or even with the aid of a lighthouse in heavy fog.

British Patent No. 1,429,743 discloses an apparatus for transmitting bearing information including two fixed omnidirectional aerials spaced a predetermined distance apart, means for transmitting from one aerial a composite signal comprising a combination of a VHF carrier frequency periodically modulated by an audio frequency and the unmodulated carrier such that the modulated and unmodulated carrier frequencies are in antiphase, means for transmitting from the second aerial the periodically modulated carrier only, means for cyclically varying the phase of one of the transmitted signals relative to the other transmitted signal, and means for distinctively altering the audio frequency modulation of the carrier frequency at the commencement of each cyclic variation in phase of the one transmitted signal.

Such an apparatus will transmit signals which can be picked up by any VHF communications receiver, however humble.

The transmission creates in effect a radio vector rotating slowly about a fixed point, i.e. the site where the two aerials are located. This vector is defined, to the VHF radio listener, by a point in time when the received signal has a minimum audio frequency modulation. The listener will hear first a distinctive signal which serves to identify the particular shore station being received and the start of a period time. He then hears a series of equally spaced clearly distinguished audio signals. Beginning at the end of the distinctive timing signal the listener counts the number of regularly spaced audio signals until the audio frequency modulation reaches a null. The count so obtained can be directly translated into a bearing from the shore station. e.g. by reference to a table.Thus with the aid of no more than a VHF radio receiver, a bearing table (which could be memorised) and a chart (which could also be memorised) a sailor can determine his bearing from a fixed point on land and navigate a safe channel along that bearing.

The modulation cycle described in British Patent No. 1,472,239 is one in which the audio frequency signal can be amplitude modulated, e.g. as a Morse Code group, prior to a short period of uninterrupted audio frequency modulation of the carrier.

The Morse code group identifies the station to which the receiver is tuned.

British Patent No. 1,472,239 further discloses apparatus of the above type in which the means for distinctively altering the audio frequency modulation at the commencement of each cycle includes means for modulating the VHF carrier frequency with a coded audio frequency signal. An example of such distinctive alteration of the audio frequency modulation is where the duration of the various periods of audio frequency modulation and silence can be adjusted to provide further information to assist the listener.

A typical transmission cycle could be as follows: Switch on, C1, P, C2, N, C3, M, Switch off, in which C1, C2 and C3, are periods of carrier only, P is a period of audio modulation, N is the total number of audio bleeps in the cycle and M is the Morse code group.

C1 may be typically 1 second in duration for all the stations. The duration of P indicates the 'sequence' of the station. This is required where the station is one of a number time sharing a common transmission channel. Thus if 4.1 sec < P < 4.4 sec this signifies that the station is transmitting in time slot 1 whereas if 4.5 sec < P < 4.9 sec this signifies that the station is in time slot 2 and so on. C2 has a duration which indicates whether the transmitter equipment in use is a main equipment of a standby equipment; This will enable service personnel to determine from the radiated signal whether servicing or repairs are required without the necessity of actually visiting the site, which may be remote or have difficult access. After the N audio bleeps the duration of the period of silence C3 indicates the system granularity.

For example if 1.0 sec < C3 < 1.3 sec the granularity is 0.25 per audio bleep whereas if 1.4 sec < C3 < 1.7.sec the granularity is 0.5 and so on.

Finally the Morse code group is transmitted then the transmitter is switched off.

In addition the frequency of the audio signal can be used to indicate the bearing of the station centre line, i.e. the direction of the radial at N/2. If, for example, the audio frequency is 1 KHz the centre line is due North, if it is 1.360 KHz the bearing is due East and so on. This uses the frequency band 1 KHz t0 2.44 KHz which is within the receiver pass-band and gives a 4 Hz change in frequency per degree change in bearing.

It will be appreciated that to decode all this information the listener cannot rely on his ears alone. Additional equipment must be connected to the VHF receiver. This additional equipment comprises conventional filters, logic circuits, timing circuits and frequency measuring circuits to time the various periods of modulation and silence and some form of display to give the navigator the decoded information. The design of such equipment is well within the grasp of those skilled in the art of electronics and forms no part of the present invention.

However, problems arise when designing.

the additional equipment to time the various periods of modulated and unmodulated carrier, especially when it has to handle received signals which are somewhat noisy.

According to the present invention there is provided an apparatus of the type disclosed in British Patent No. 1,472,239 including means for transmitting at a predetermined time or times in the modulation cycle a group or groups of binary coded signals.

The term "binary coded signals" is used herein to denote a sequence of contiguous two-state code elements of equal durations which can subdivide into fixed size groups each representing a character or the like in a message according to a mathematical encoding law. In contrast thereto Morse coded signals can be defined as a sequence of three-state code elements of unequal durations which are subdivided into varying sized groups each representing a character according to any arbitrary encoding table.

Although it is difficult to design logic equipment which under noisy conditions will measure accurately periods of audio modulated or unmodulated carrier, it is relatively easy to design logic circuits which can with a high degree of correlation recognise high speed binary coded signals which are not capable of recognition by a listener.

Thus to make a station of the type described above suitable for use by both human listeners and automatic equipment it is desirable to incorporate in transmission sequence signals suitable for both types of operation. So far as automatic equipment is concerned the main difficulty arises with synchronising the equipment. Once synchronization has been established at the start of the cycle the equipment can then time the subsequent periods satisfactorily. If however the signals are noisy the automatic equipment may not be able to establish a correct synchronization from the initial period P of audio modulation although the listener can distinguish it.

The brief, unique binary signal which now precedes P is designed so that it can be unambiguously recognised by a logic circuit, even if the signals are noisy. Recognition of this signal is used to set the automatic equipment synchronization.

Another problem which may arise is a malfunction in the transmitting station.

Since each station is only concerned with repetitive transmission of the same cycle of signals it is easy to monitor the signals that are transmitted. If a malfunction occurs the listener may not have any great difficulty in recognising that something is wrong. But automatic equipment is not so perceptive and may well provide false readings. The station is therefore arranged to monitor the transmission of each cycle and to transmit at the end of the cycle a "validating" flag signal if the preceding signals in the cycle are satisfactory. If the transmission is faulty then the flag is not transmitted and its absence causes the automatic equipment of the receiver to emit a suitable warning or alarm.Conveniently the flag signal is also a high speed binary code group, common to a number of stations, rather than the morse identity which is difficult to recognise at the receiver since it differs between the transmitter stations.

WHAT WE CLAIM IS: 1. Apparatus of the type disclosed in British Patent No. 1,472,239 for transmitting bearing information including means for transmitting at a predetermined time or times in the modulation cycle a group or groups of binary coded signals.

2. Apparatus according to claim 1 wherein said means transmits a unique synchronising binary group immediately prior to a timed period of audio frequency modulation.

3. Apparatus according to claim 1 or 2 including means for monitoring the signals transmitted during the cycle, said transmitting means being arranged to transmit a unique validation binary group at the end of the cycle if the preceding signals in the cycle are determined by the monitoring means to be satisfactory.

4. Apparatus for transmitting bearing information substantially as hereinbefore described.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. recognising that something is wrong. But automatic equipment is not so perceptive and may well provide false readings. The station is therefore arranged to monitor the transmission of each cycle and to transmit at the end of the cycle a "validating" flag signal if the preceding signals in the cycle are satisfactory. If the transmission is faulty then the flag is not transmitted and its absence causes the automatic equipment of the receiver to emit a suitable warning or alarm. Conveniently the flag signal is also a high speed binary code group, common to a number of stations, rather than the morse identity which is difficult to recognise at the receiver since it differs between the transmitter stations. WHAT WE CLAIM IS:

1. Apparatus of the type disclosed in British Patent No. 1,472,239 for transmitting bearing information including means for transmitting at a predetermined time or times in the modulation cycle a group or groups of binary coded signals.

2. Apparatus according to claim 1 wherein said means transmits a unique synchronising binary group immediately prior to a timed period of audio frequency modulation.

3. Apparatus according to claim 1 or 2 including means for monitoring the signals transmitted during the cycle, said transmitting means being arranged to transmit a unique validation binary group at the end of the cycle if the preceding signals in the cycle are determined by the monitoring means to be satisfactory.

4. Apparatus for transmitting bearing information substantially as hereinbefore described.